Recently, on operation spots where concrete is used, such as civil engineering spots or construction spots, vibration compaction operation is used which generates noises and hence there has been an increasing demand for concrete that requires no compaction operation in order to reduce noises and decrease work load on the spot as well. Furthermore, with increasing complexity and functionality of constructions, it is necessary to pack concrete uniformly into inside the frames where reinforcements are densely distributed. In this regard, extensive studies have been made on the development of concrete that has flowability, flowability retaining ability, high packing property, and resistance to segregation and needs no compaction operation.
Generally, a cement dispersant is added to concrete in order to improve the flowability, flowability retaining ability and packing property thereof. Such a cement dispersant includes various substances, e.g., melaminesulfonic acid formaldehyde salts, naphthalenesulfonic acid formaldehyde salts, polycarboxylic acid salts, etc.
However, these cement dispersants when added to concrete can impart flowability and flowability retaining ability to concrete due to their high dispersing effect but cause the phenomenon that concrete loses uniformity during its flow and the components of aggregate will separate, thereby deteriorating the feasibility of concrete. Furthermore, after hardening, the concrete has an inferior quality (see, for example, JP-A-5-238795 (the term "JP-A" as used herein means an "unexamined published Japanese patent application), and JP-A-5-298795, JP-A-6-225354 (U.S. Pat. No. 5,362,829)).
With view to inhibiting or suppressing the segregation, there has been studied addition of water-soluble polymers, for example, cellulose derivatives such as methyl cellulose ether, synthetic polymers such as polyacrylamide, natural polysaccharides such as cardlan as a segregation reducing agent to concrete (see, for example, JP-A-6-206752, JP-A-9-132444, and JP-A-9-2856). However, the water-soluble polymers as a segregation reducing agent have the problem that addition in an amount sufficient to inhibit the segregation results in an increase in viscosity of concrete and the flowability and flowability retaining ability of concrete are decreased.
As a segregation reducing agent that does not deteriorate the flowability and flowability retaining ability of concrete even in a concentration sufficient to suppress the segregation, there has been proposed in JP-A-10-53627 copolymers containing a N-vinylcarboxamide monomer. However, to impart concrete with flowability, flowability retaining ability, high packing property and resistance to segregation, it is necessary to add to concrete the above-mentioned cement dispersant and a segregation reducing agent consisting of copolymer containing a N-vinylcarboxamide monomer in combination.
As described above, in the prior art, it has been impossible to impart concrete with flowability, flowability retaining ability, high packing property, and resistance to segregation which are required for concrete by means of a single agent of copolymer in good balance.